ECOLOGY

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ECOLOGY

• Primary Production and Energy Flow

How do I become more productive?
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Ecosystem Ecology

• The term ecosystem first proposed by Arthur
  Tansley (1935)
• Though the organisms may claim our primary
  interest,we cannot separate them from their
  special environment, with which they form one
  system. It is the ecosystems so formed which,
  from the point of view of the ecologist, are the
  basic units of nature on the face of the earth.
• Ecosystem Ecology The study of energy, water
  and nutrient flows (flux) in ecosystems
• Biotic and abiotic processes
• Fundamental areas of interest include primary
  production, energy flow and nutrient cycling

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Definitions

• Population a collection of individuals of the
  same species
• Community an assemblage of populations in an
  area or habitat
• Ecosystem a biological community plus all of
  the abiotic factors influencing that community

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Definitions

• Production creation of new organic matter
• Primary Production (PP) fixation of energy by
  autotrophs
• PP measured in various ways
• usu. rate of CO2 uptake (g/m2yr1)
• Or -- biomass or O2 produced
• Gross Primary Production (GPP) total amount of
  energy fixed (or CO2 taken up) by all autotrophs
  in an ecosystem.
• Net Primary Production (NPP) amount of energy
  left over after autotrophs have met their own
  energetic needs (respiration and, ie, the amount
  of energy available to consumers).
• NPP GPP RPP

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Trophic Dynamics

• Lindeman (1942) The Trophic Dynamic Aspect of
  Ecology
• Trophic dynamics transfer of energy from one
  part of an ecosystem to another
• First suggested grouping organisms within an
  ecosystem into trophic levels.
• The number of trophic levels in an ecosystem is
  limited by energy losses with each transfer or
  conversion of energy between trophic levels

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Trophic Dynamics

• Each T.L. feeds on T.L. immediately below.
• As energy is transferred from one T.L to another,
  energy is degraded/lost due to
• Limited assimilation
• Consumer respiration
• Heat production
• Energy quantity decreases with each successive
  trophic level

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A Simplified Food Web
Secondary Consumers (Predators)
Primary Consumers (Herbivores)
Primary Producers (Plants)
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A Simplified Food Web
Secondary Consumers (Predators)
PAR
Primary Consumers (Herbivores)
Primary Producers (Plants)
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A Simplified Food Web
Secondary Consumers (Predators)
PAR
Primary Consumers (Herbivores)
reflected
Primary Producers (Plants)
absorbed by Chl a
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A Simplified Food Web
Secondary Consumers (Predators)
PAR
Primary Consumers (Herbivores)
reflected
Primary Producers (Plants)
photorespiration heat production
absorbed by Chl a
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A Simplified Food Web
Secondary Consumers (Predators)
PAR
Primary Consumers (Herbivores)
reflected
Primary Producers (Plants)
photorespiration heat production
absorbed by Chl a
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A Simplified Food Web
Secondary Consumers (Predators)
PAR
respiration heat production
Primary Consumers (Herbivores)
limited assimilation
reflected
Primary Producers (Plants)
photorespiration heat production
absorbed by Chl a
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A Simplified Food Web
Secondary Consumers (Predators)
PAR
respiration heat production
Primary Consumers (Herbivores)
limited assimilation
reflected
Primary Producers (Plants)
photorespiration heat production
absorbed by Chl a
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Hence the Trophic Pyramid
Biomass and productivity generally decrease with
trophic level
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Energy Flow In A Temperate Deciduous Forest

• Gosz et al. (1978) study of solar energy flux at
  Hubbard Brook experimental forest
• Of the total energy input via solar radiation
• 15 reflected
• 41 converted to heat.
• 42 absorbed during evapotranspiration.
• 2.2 fixed by plants as GPP
• 1.2 used in plant respiration.
• 1 left for NPP

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Energy Flow In A Temperate Deciduous Forest

• Gosz et al. 1978
• lt 1 of total energy input converted to NPP
• Of the NPP available to consumers (herbivores),
  96 lost as consumer respiration
• Insufficient energy left to support a viable
  population at a 3rd trophic level.

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Factors Controlling Terrestrial PP

• PP variable but most strongly correlated with
  temperature and moisture.
• Highest PP under warm, moist conditions
• An indicator of PP is actual evapotranspiration
  (AET)
• Total amount of water that evaporates and
  transpires off a landscape during the course of a
  year (mm H2O/yr)
• affected by temp and precipitation
• ecosystems with high AET tend to be warm, receive
  large amounts of precipitation
• ecosystems with low AET tend to be cold, receive
  little precipitation or both
• eg hot deserts and cold tundra have low AET

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• Among different types of ecosystems, AET is
  correlated with NPP
• Rosenzweig (1968) estimated influence of moisture
  and temperature on rates of PP by plotting the
  relationship between annual NPP and AET.

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Factors Controlling Terrestrial PP

• Within similar ecosystems, temps tend to be
  similar, so moisture (precipitation) tends to be
  the controlling factor

• eg Sala et al. (1988) study of 9,498 sites in the
  grasslands of central North America

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Factors Controlling Terrestrial PP

• Patterns of soil fertility also explain
  significant variation in NPP within terrestrial
  ecosystems (assuming equal temp. and moisture)
• Liebig (1840) concept of nutrient limitation

• Nutrient availability controls patterns of PP in
  agricultural ecosystems...

NB Liebigs law somewhat simplistic 2 or more
factors may be simultaneously limiting in many
systems
Liebigs Law of the Minimum
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Factors Controlling Terrestrial PP

• ...and in natural ecosystems
• Shaver and Chapin (1986)
• Added commercial fertilizer (NPK) to several
  tundra systems in Alaska
• NPP 23 30 higher on fertilized plots.

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Factors Controlling Terrestrial PP

• Effects of nutrient additions depend on prior
  nutrient availability
• PP responds to additions of limiting nutrients
• eg Bowman et al. (1993)
• Experimental fertilization of wet and dry alpine
  meadows, Niwot Ridge, CO.
• Wet meadows had higher initial N, P
• 4 treatments
• N P NP control

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Factors Controlling Terrestrial PP

• Bowman et al. (1993)
• Results more dramatic in dry meadow (lower
  initial N, P)
• N, NP both produced signif. ? biomass
• In contrast, weaker response in wet meadow
• only NP had an effect
• sugg. N-limitation in dry meadow co-limitation
  in wet meadow
• Light might also limit NPP in wet meadow ?
  biomass might produce enough shading to inhibit
  growth response to nutrient additions.

statistically significant response
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Factors Controlling Aquatic PP

• Phytoplankton are the dominant primary producers
  in aquatic ecosystems
• Aquatic NPP generally limited by nutrient
  availability
• temperatures generally less variable in the ocean
  than on land
• Several studies have found proportional
  relationship between P and phytoplankton
  biomass, chlorophyll a and NPP in lakes.

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Observational studies in Japan, North America
eg Hogetsu and Ichimura 1954 Ichimura
1956 Sakamoto 1966 Dillon and Rigler 1974 Smith
1979
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Lake Fertilization Experiments

• eg. studies at Experimental Lakes Area, Ontario
  (Mills and Schindler 1987, Findlay and Kasian
  1987)

• Lake 226 divided by vinyl curtain
• Each basin 8 ha, 500,000 m3
• One side fertilized with P

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Marine NPP is also controlled by nutrient
availability

• Highest rates of NPP in areas with greatest
  nutrient availability
• continental margins (runoff from land,
  bioturbation of bottom sediments)
• areas of upwelling (deep nutrient-laden waters
  rise to euphotic zone)
• Open ocean tends to be nutrient poor (relies on
  vertical mixing for nutrients)

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Unlike lakes, marine NPP appears to be limited
primarily by N

• Granéli et al. (1990) nutrient enrichment
  experiments, Baltic Sea
• Fertilized flasks containing indigenous
  phytoplankton spp.
• N, P, control treatments
• N treatments led to increased chlorophyll
  concentrations.

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Factors Controlling NPP

• Temperature and precipitation (terrestrial) and
  nutrient availability (aquatic) explain most of
  the variation seen in NPP, but not all
• Residual Variation proportion of variation not
  explained by the independent variable.
• Dillon and Rigler (1974) suggested environmental
  factors besides nutrient availability
  significantly influence phytoplankton biomass.

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Summary

• Terrestrial Primary Production is generally
  limited by temperature and moisture.
• Aquatic Primary Production is generally limited
  by nutrient availability.
• usually P in freshwater ecosystems
• N in marine ecosystems
• Energy losses limit the number of trophic levels
  found in ecosystems.